The work described herein consists of an examination by cDNA complexity analysis of the changes that occur in the total mRNA population of rooster liver following treatment with 17beta-estradiol. It has been demonstrated that the hormone markedly affects the concentration of no more than approximately 200 RNA species and that its major effect is the induction of a few very abundant species of mRNA. It has been possible to isolate a cDNA probe that is specific for these sequences and which will provide a suitable means of selecting clones containing estrogen-inducible sequences.